The present invention relates generally to circuits used to measure capacitance, and more particularly to a method for measuring the capacitance of a backup power supply in a restraint control module.
Over the past several years, there has been a trend in the automotive industry to install air bag systems in vehicles to enhance protection traditionally afforded vehicle occupants through the use of seat belts. In some automotive vehicles, the inflatable restraint system may be an air bag system disposed within an occupant compartment in the automotive vehicle in close proximity to either a driver occupant or one or more passenger occupants. Usually, the air bag system includes a restraint control module that is connected to the vehicle structure and an air bag for deployment through an opening in a cover to extend and inflate in an occupant compartment of the automotive vehicle. The deployed air bag restrains movement of the occupant to protect the occupant from forcefully hitting parts of the automotive vehicle as a result of an automobile accident.
Air bag systems typically include a restraint control module, a reaction canister and an air bag and inflator that are stored inside the reaction canister. Generally speaking, the inflator is actuated by a signal received from a vehicle deceleration sensor or accelerometer that is connected to the restraint control module, which, in turn, causes a discharge of inflator gas into the interior of the air bag. The restraint control module controls the overall operation of the air bag system and essentially could be viewed as the main control unit for the air bag system.
As with any system based on electronic components and sensors, air bag systems and their associated electronic components, require power from a power supply in order to function properly. During normal operation, the power used to drive an air bag system and its related components originates from a battery that is located in the automotive vehicle. During some automotive accidents, the electrical conductors that connect the battery to the air bag system and its related electronic components may become severed or damaged so that power is cut off to these systems. As a result of the conductor being severed or damaged, the electronic components of the air bag system lose power and will not function properly due to the loss of power.
Due to this potential loss of power, a backup power supply for a restraint control module can be added to the air bag system that is capable of providing power during times in which the main power source is not capable of providing power. In the automotive setting, the backup power source will be required to operate over extended periods of time and over a wide range of operating conditions. As a result, a need exists to ensure that the backup power source is functioning in the system so that if needed, the backup power source will be capable of providing adequate power to deploy the air bags.
The present invention discloses a method of measuring the capacitance of a backup power source in a backup power supply system using a capacitance measurement system. In the preferred embodiment of the present invention, during normal operation a regulated output voltage is generated by the backup power source and is set to a nominal regulated output voltage. Once set to the nominal regulated output voltage, the backup power supply system is adjusted by the capacitance measurement system to regulate the voltage provided by the backup power source to a predetermined regulated output voltage. In the preferred embodiment, the predetermined regulated output voltage is set 1 V below the nominal regulated output voltage. As set forth in the detailed description below, this is one of the values that is used by a main control unit to calculate the capacitance of the backup power source.
In addition to setting the regulated output voltage to the predetermined regulated output voltage, a first predetermined load is connected to the backup power source to dissipate energy that is stored in the backup power source. As the first predetermined load is connected to the backup power source, a counter is started that generates a first count value that corresponds to the amount of time that it takes for the backup power supply system to start regulating the regulated output voltage generated by the backup power source at the predetermined regulated output voltage. In the preferred embodiment, the backup power supply system is used to generate a regulated output voltage that is larger than its respective input voltage. The preferred nominal regulated output voltage and the predetermined regulated output voltage are also larger than the input voltage to the backup power supply system. As set forth in the detailed description, the change in voltage from the nominal regulated output voltage setting to the predetermined regulated output voltage setting is also used to calculate the capacitance of the backup power source.
Once the regulated output voltage reaches the predetermined regulated output voltage, the counter is stopped and the first count value is stored. Preferentially, the first count value is stored in a register that is connected to a main control unit; however, the first count value may also be stored in the main control unit in alternative embodiments. After the first count value is obtained, the regulated output voltage that is available on the backup power source is set back to the nominal regulated output voltage by the capacitance measurement system. Once the backup power source reaches the nominal regulated output voltage and is functioning properly, the backup power supply system is once again adjusted by the capacitance measurement system to regulate the voltage provided by the backup power source to the predetermined regulated output voltage. Next, a second predetermined load is connected to the backup power source to once again dissipate the energy stored in the backup power source.
As with the prior operation, the counter is also started to generate a second count value corresponding to the amount of time that it takes for the backup power supply system to start regulating the regulated output voltage generated by the backup power source at the predetermined regulated output voltage. Once the backup power supply system begins regulating at the predetermined regulated output voltage or in other words, reaches the predetermined regulated output voltage, the counter is stopped and the second count value is stored. As set forth in the detailed description, the second count value is also used to calculate the capacitance of the backup power source. Once the first count value and the second count value have been obtained, the capacitance of the backup power source is capable of being calculated by a main control unit.
In the preferred embodiment, a voltage set point circuit is used to adjust the backup power supply system to regulate the voltage provided by the backup power source to the predetermined regulated output voltage. The main control unit and a capacitance measurement control and status circuit are connected to the voltage set point circuit for controlling the voltage set point circuit. In the preferred embodiment, the main control unit is connected to a load select circuit that is used to connect the first and second predetermined loads to the backup power source during a capacitance measurement cycle. In addition, the capacitance measurement control and status circuit is connected to a state machine that controls the counter. An oscillator is connected to the state machine for driving the counter, the oscillator being set to a predetermined frequency that is also used in calculating the capacitance of the backup power source.
The first count value and the second count value are stored in a register that is connected to the main control unit and the output of the counter in the preferred embodiment of the present invention. The capacitance measurement system is also capable of being reset between measurements or counting cycles and in fact, in the preferred embodiment of the present invention the counter is reset after each counting cycle. However, those skilled in the art would recognize that the counter does not necessarily have to be reset after each counting cycle, such that, the last count is subtracted from the current count to obtain the actual count for each cycle. Those skilled in the art would recognize that the counter could be reset for various other reasons as well, such as when the counter experiences an overflow error.
A regulated output voltage sensor is connected to the backup power supply system and is used to generate a predetermined output signal that is used to stop the counter. In the preferred embodiment, the output of the regulated output voltage sensor is connected to the capacitance measurement and control circuit, which in turn, stops the counter upon receiving a predetermined output signal from the regulated output voltage sensor. The preferred regulated output voltage sensor comprises a reference voltage, a current source, a DMOS transistor and an inverter. For reasons set forth in detail below, in the preferred embodiment of the present invention the regulated output voltage sensor is connected to a boost converter switching device, which is used together with a backup power source charging circuit to regulate the voltage level present in the backup power source.
The capacitance measurement control and status circuit is capable of controlling the capacitance measurement system and providing a plurality of predetermined status signals to the main control unit. The main control unit continuously monitors the status of the capacitance measurement system during operation. In the preferred embodiment, the capacitance measurement control and status circuit is capable of notifying the main control unit that the capacitance measurement system is ready to begin taking a measurement, has experienced an error during a measurement and whether or not a measurement or count is underway, for example. The preferred capacitance measurement control and status circuit comprises a first OR gate, a first flip-flop, a first AND gate, a second AND gate, a second OR gate and a second flip-flop. Preferentially, the outputs of each flip-flop is used to generate status signals during operation of the capacitance measurement system, which are in turn added to the output of the counter as the two most significant bits of a 16-bit output signal.
Another embodiment of the present invention discloses a capacitance measurement system for a backup power supply system having a regulated backup power source that is capable of providing an adjustable regulated output voltage. The capacitance measurement system includes a voltage set point circuit that is connected to the backup power supply system for setting the backup power supply system to operate at a lower regulated output voltage than a nominal regulated output voltage. A capacitance measurement control and status circuit is connected to the voltage set point circuit for controlling operation of the voltage set point circuit. A counter is connected to the capacitance measurement control and status circuit, which is also controlled by the capacitance measurement control and status circuit.
A regulated output voltage sensor is connected to the backup power supply system for determining when the backup power source reaches the lower regulated output voltage during a capacitance measurement test. In the preferred embodiment, this is accomplished by monitoring when the backup power supply system begins regulating the output voltage on the backup power source in the preferred embodiment of the present invention. The preferred backup power supply system generates a regulated output voltage on the backup power source that is larger than its input voltage. In addition, the preferred predetermined regulated output voltage is less than the nominal regular output voltage.
During the capacitance measurement test, the backup power source is set to operate at a lower regulated output voltage than the nominal regulated output voltage by the voltage set point circuit. In addition, a first predetermined load is applied to the backup power source to lower the regulated output voltage from the nominal regulated output voltage to a predetermined lower regulated output voltage. The counter is started to generate a first count value that corresponds to the amount of time that it takes to lower the regulated output voltage from the nominal regulated output voltage to the lower regulated output voltage, at which point the backup power supply system begins regulating the backup power source to the lower regulated output voltage and the counter is deactivated by the signal from the regulated output voltage sensor. In the preferred embodiment, the output of the counter is stored in a register or some other equivalent memory storage device.
After the first count value is obtained, the backup power source is set to operate at the nominal regulated output voltage, which causes the backup power supply to boost the regulated output voltage back up to the nominal regulated output voltage. Once the nominal regulated output voltage is reached, the backup power source is once again set to operate at the lower regulated output voltage by the voltage set point circuit. At this point, a second predetermined load is applied to the backup power source to lower the regulated output voltage from the nominal regulated output voltage to the lower regulated output voltage. In addition, the counter is started to generate a second count value corresponding to the amount of time that it takes to lower the regulated output voltage from the nominal regulated output voltage to the lower regulated output voltage.
Once the regulated output voltage of the backup power service reaches the lower regulated output voltage, the counter is stopped with an output signal from the regulated voltage sensor. In the preferred embodiment, the regulated voltage sensor senses when the backup power supply system begins regulating the voltage on the backup power source to determine when the regulated output voltage reaches the lower regulation output voltage After the second count value has been obtained, the first count value and the second count value, together with several other known variables, can be used to calculate the capacitance of the backup power source of the backup power supply system.
Further objects and advantages of the present invention will be apparent from the following description, reference being made to the accompanying drawings wherein preferred embodiments of the present invention are clearly shown.